The present invention relates generally to dispersion compensators that are employed in optical communication systems, and more particularly to a dispersion compensator that employs Bragg gratings operating in transmission.
Optical wavelength division multiplexing (WDM) has gradually become the standard backbone networks for fiber optic communication systems. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology.
One phenomenon that has an adverse effect on the quality of a WDM optical-signal is chromatic dispersion, in which the index of refraction of the transmission medium is dependent on wavelength. Chromatic dispersion causes the different wavelengths of a signal to undergo different phase shifts, resulting in spreading or broadening of the signal, which can give rise to transmission errors.
One dispersion compensation approach employs a Bragg fiber grating, which comprises a length of optical fiber having a series of perturbations in the refractive index that are spaced along the fiber length. The gratings are sometimes classified by the distribution of the index perturbations along the fiber axis. For example, certain fiber gratings may be classified as uniform, in which the perturbations are equally spaced from one another, chirped, in which the spacing between successive perturbations decreases, or apodized, in which the magnitude of the perturbations vary in accordance with some function of position along the fiber.
Conventional fiber Bragg gratings are typically fabricated by providing an optical fiber with one or more dopants sensitive to ultraviolet light, such as fibers having cores doped with germanium oxide, and exposing the fiber at periodic intervals to high intensity ultraviolet light from an excimer laser. The ultraviolet light interacts with the photosensitive dopant to produce long-term perturbations in the local index of refraction. The appropriate periodic spacing of perturbations to achieve a conventional grating can be obtained by use of a physical mask, a phase mask, or a pair of interfering beams.
Conventional dispersion compensators often employ chirped fiber grating operating in a reflection mode so that the different wavelengths in the signal are reflected at different positions along the grating, causing the different wavelengths to experience different path lengths. FIG. 1 schematically depicts such a dispersion compensator. In operation, an input signal 20 enters input port 23 of a circulator 22, exits circulator port 24, propagates towards chirped grating 11, from which it is reflected, enters circulator port 24, and exits the circulator at port 25. If the distances between the index perturbations in the grating are properly matched to the wavelengths of the optical signal, the resulting wavelength-dependent delays will negate the dispersion in output signal 21.
While the results from dispersion compensators employing reflection-based fiber gratings are typically satisfactory, they have a number of disadvantages. In particular, such dispersion compensators require components external to the fiber itself such as the aforementioned circulator. This gives rise to additional losses beyond those .inherent in the fiber and also prevents the device from being easily integrated on a planar waveguide.
Dispersion compensators using fiber gratings in transmission have been proposed to overcome some of the problems of fiber gratings used in reflection. For example, a transmission-based dispersion compensator using an apodized, unchirped fiber Bragg grating is discussed in N. M. Litchinitser, B. J. Eggleton, and D. B. Patterson, J. of Lightwave Technology 15, no.8, pp. 1303-1313 (1997), and K. Hinton, J. of Lightwave Technology 16, no.12, pp. 2336-2346 (1998). Such a dispersion compensator can overcome a number of the aforementioned limitations of dispersion compensators that employ gratings operating in reflection. For example, they are compatible with planar integrated optics technology, have relatively low insertion loss, and have no inherent group delay ripple. Nevertheless, there remains a need to develop an improved dispersion compensator that employs an apodized fiber grating operating in transmission that has performance characteristics which allow it to be used in practical optical communication systems.
In accordance with the present invention, an apparatus is provided to compensate for dispersion in a transmission medium. The apparatus includes an input port for receiving a WDM optical signal having a plurality of signal wavelengths and a first Bragg transmission grating receiving the WDM optical signal from the input port. The first Bragg transmission grating has non-zero dispersion at at least one of the signal wavelengths. The first Bragg transmission grating also has a Bragg wavelength that is chosen so that all of the plurality of signal wavelengths lie outside of a reflection band of the first Bragg transmission grating. A second Bragg transmission grating, which is optically coupled to the first Bragg transmission grating, has a non-zero dispersion at at least one of the signal wavelengths. The second Bragg transmission grating also has a Bragg wavelength that is selected so that all of the plurality of signal wavelengths lie outside of a reflection band of the second Bragg transmission grating. Finally, an output port is provided for receiving the optical signal from the second Bragg grating and communicating the optical signal to an external source.
In accordance with one aspect of the invention, the first and second Bragg transmission gratings may be unchirped, apodized gratings.
In accordance with another aspect of the invention, a method is provided for compensating for dispersion that arises in an optical transmission path. The method begins by receiving an optical signal traveling along the transmission path. The optical signal is successively transmitted through a plurality of Bragg gratings that each have non-zero dispersion at at least one signal wavelength of the optical signal.
In accordance with another aspect of the invention, a tunable dispersion compensator is provided for compensating for dispersion in a transmission medium. The dispersion compensator includes an input port for receiving a WDM optical signal having a plurality of signal wavelengths and a tunable Bragg transmission grating having a selectively adjustable value of dispersion that receives the WDM optical signal from the input port. The tunable Bragg transmission grating also has a non-zero dispersion at at least one of the signal wavelengths and a Bragg wavelength that is selected so that all of the plurality of signal wavelengths lie outside of a reflection band of the Bragg transmission grating. An output port is provided for receiving the optical signal from the tunable Bragg grating and for communicating the optical signal to an external source.